Currently, there exists a lack of effective diagnostic methods for the detection of inter-phase moisture ingress defects within cable bodies. While defect location techniques based on frequency domain reflectometry (FDR) and time domain reflectometry (TDR) restoration enable precise localization and polarity determination of fault points, they struggle to effectively identify specific types of defects, resulting in high misdiagnosis rates. To address this issue, a novel method for locating and diagnosing inter-phase moisture ingress in distribution cables is proposed, leveraging dynamic frequency domain reflectometry. By applying temperature stimuli, dynamic changes are induced in the distributed capacitance parameters of the affected moisture-laden segments. By integrating the amplitude and waveform variations observed on frequency domain location maps and time domain restoration plots in response to temperature changes, we achieve the localization and diagnosis of inter-phase moisture ingress defects within cable bodies. Experimental validation on actual cables has demonstrated that following the ingress of water and moisture between inter-phases within distribution cables, the distributed capacitance of the affected segments is lower than that of the normal segments. Under temperature stimuli, the distributed capacitance of the moisture-laden segments increases with temperature, gradually approaching that of the normal segments, while the reflection peak amplitude on the frequency domain location maps decreases. Simultaneously, the time domain restoration waveform maintains a “left positive, right negative” pattern with a decreasing amplitude.